Conventionally, a high-temperature, high-pressure pressing machine employed in producing special ceramics and hard metals has a press cylinder. The press cylinder has a pad, an artificial graphite-made outer casing placed on the pad, an artificial graphite-made inner casing, generally known as a sleeve, fitted in the outer casing in contact with the inner periphery of the outer casing, a pair of artificial graphite-made press plates fitted in the inner casing, and a press bar fitted in the inner casing. One of the press plates is placed on the pad, and the other press plate which is placed above the one press plate is pressed by the press bar from above. In sinter molding ceramics and hard metals by such high-temperature, high-pressure pressing machine, the material to be sinter molded is loaded between the two press plates, and same is pressed by the press bar under a pressure of more than 100 kg/cm.sup.2 under vacuum or in a non-oxidizable atmosphere, such as argon or nitrogen, then heated to more than 1000.degree. C. by induction heating or electric resistance heating using an artificial carbon material. The inner casing may be discarded and replaced by a new one if sintered ceramics and/or metals have deposited on its inner wall, whereby ceramic/metal depositions on the inner periphery of the outer casing is prevented so as to permit prolonged use of the outer casing.
Hitherto, no material other than artificial graphite has been available which can withstand high pressure at high temperatures of more than 1000.degree. C. Recently, however, there are needs for a high-temperature, high-pressure pressing machine which can withstand still higher pressure conditions. In order to meet the needs, it has been proposed to increase the wall thickness of the artificial graphite-made outer casing. However, such approach is still insufficient to provide the desired strength characteristics, and if the desired strength is to be met, the wall thickness must be considerably increased, which naturally means increased weight or inconvenience in handling, and decreased internal space or decreased capacity for housing the material to be sinter molded, that is, lower efficiency.
Attempts have been made to overcome this problem by using a carbon fiber reinforced carbon composite (hereinafter referred to as CRC), a recently developed material, for the outer casing of a press cylinder in a pressing machine of the above mentioned type. This material, CRC, is lower in specific gravity and has several times higher bending, tensile, and impact strength characteristics than artificial graphite material of the conventional type. Furthermore, it exhibits remarkably better high-temperature characteristics in a non-oxidizable atmosphere than other materials. For these reasons it is used for missile and rocket parts, aircraft brakes, and the like.
Fabrication of a cylindrical outer casing using CRC is usually carried out according to the following procedure. Carbon filaments impregnated with a phenolic, epoxy, or furan resin are wound on a metallic cylinder of a specified diameter to the desired width (or length of the outer casing) and thickness, and same is subjected to the step of hardening reaction at temperatures of 100.degree. C. to 200.degree. C. Then, the metallic cylinder is removed to give a cylindrical body, which is then calcined at a temperature of more than 1000.degree. C. Subsequently, steps of impregnation with the phenolic, epoxy, or furan resin and calcination are repeated several times.
The circumferential tensile strength of a CRC cylindrical body is of the order of 2500 kg/cm.sup.2 or about ten or more times that of a conventional artificial graphite material. Now, the strength of an outer casing of a press cylinder of a high-temperature, high-pressure pressing machine and the internal pressure to which it can withstand have a relationship expressed by the following equation, and where the outer casing has a higher circumferential tensile strength, a higher internal pressure can be applied: EQU .delta.tmax=P(.gamma.1.sup.2 +.gamma.2.sup.2)/(.gamma.1.sup.2 -.gamma.2.sup.2)
(where .delta.t: circumferential tensile strength, P: internal pressure, .gamma.1: outer radius, .gamma.2: inner radius) Therefore, by employing a CRC outer casing it is possible to carry out sinter molding of ceramics or alloys under high-temperature, high-pressure conditions by applying a higher pressure than in the case of a conventional high-temperature, high-pressure pressing machine. Accordingly, it is known that use of such outer casing provides various advantages: that a higher density sintered material can be obtained; and that the wall thickness of the outer casing can be reduced, which means light weight, increased inner diameter, increased capacity, and increased productivity.
However, the coefficient of linear expansion of CRC is smaller than that of conventional artificial graphite material. Therefore, when an artificial graphite-made inner casing fitted in a CRC outer casing, and two press plates and a press bar are subjected to dimensional increase as a result of thermal expansion under a temperature rise, compressive stress is exerted on the CRC outer casing which is less subject to thermal expansion and, if such compressive stress exceeds a certain limit, the CRC outer casing may become fractured. Another trouble is that the outer casing is subject to plastic deformation under the compressive stress and, as a result, the outer casing diametrically expands, so that when the outer casing is reused, an increased gap caused between it and the inner casing will hamper precision working. As such, despite the aforesaid advantages of CRC, attempts to use a CRC outer casing have been unsuccessful.